Hisao Iwase

Defect-free fabrication for single crystal silicon substrate by chemo-mechanical grinding

Abstract IC chips are built on Si substrates which must have a high degree of crystalline perfection. The single crystal Si ingot is first sawn into wafers, each of which then undergoes lapping, etching and several steps of polishing to remove the mechanical imperfection and to achieve mirror surfaces. An alternative process has been newly developed by effective use of solid-state reaction between the CeO 2 abrasives and Si. Si is removed in a form of amorphous Ce-O-Si at a dry condition. The fabricated ϕ300 mm Si wafers are examined on both surface and subsurface. The results show that 1) the surface is generated by fixed abrasives following grinding dynamics, 2) no defect or mechanical (structural) imperfection is introduced during fabrication and 3) far better quality is achieved than that by CMP.

Fabrication and evaluation for extremely thin Si wafer

Grinding process on the Si wafer develops subsurface damage, which remarkably degrades deflective strength of the wafer and constitutes a barrier against producing a thin wafer for low-profile packaging. In this paper, the authors propose a new index for evaluation of the Degree of Subsurface Damage (DSD). Requiring no costly instrument, the new index is easily calculated via the external geometry of the ground wafer. With the new index, it is able to quantitatively evaluate the subsurface damage introduced by different processes (or wheel) and to estimate the minimally achievable thickness of the wafer by each process. Also, a novel fixed abrasive process of Chemo-Mechanical Grinding (CMG) has been proposed for stress relief. All results indicate that the subsurface damage after CMG is nearly zero.

Microstructural Analysis for Si Wafer after CMG Process

The single crystal of Si is still one of the most important candidates among other materials including Single crystals of SiC, GaN, C(diamond) or compound semiconductors. The innovative process as called CMG(Chemo-Mechanical-Grinding) for Si wafer has been recently developed which is different from conventional CMP(Chemo-Mechanical-Polishing ) process. The CMG process can be done under dry conditions using CeO2 based solid bulk abrasives. The microstructures for surface and subsurface of Si single crystal after CMG process were analyzed using TEM/EDX, AFM, MFP-3D Microscope. The mechanism of CMG process was also investigated by X-ray diffraction and ICP chemical analysis using products by chemical reaction between Si and CeO2 abrasives. The results showed that Si single crystal after CMG had, 1) no defects even Si lattice revel or mechanical imperfections,2) better surface roughness as compared to CMP process. The CMG mechanism concluded that CeO2 reacted with Si producing Ce-Si-O amorphous phase.

Study on reaction mechanism of Si and pure CeO2 for chemical-mechanical-grinding process

The thinning process of silicon wafer for power device in automotive applications requires stress relief and relatively high Si removal rate. The innovative process of chemical mechanical grinding (CMG) has been developed for the surface finishing of Si wafer by means of solid state chemical reaction with CeO2 abrasives under dry condition. However, the reaction mechanisms of Si and pure CeO2 in the dry CMG process are yet fully understood. The chips of Si wafer produced during CMG process were analyzed using x-ray diffraction (XRD), transmission electron microscopy (TEM), and TEM/energy dispersive x-ray fluorescence spectrometer. Those analyses clearly indicated that the chips were thin, elongated, and acicular, as well as partially curved. The large amount of Si in amorphous phase and CeO2 were detected in the CMG chips by XRD, except Si crystalline. The reaction experiments between Si and CeO2 were also performed where the pellets composed of mixed Si∕CeO2 powders were heat treated at 400–1200°C in bot...

Effect of Wheel Additive On Chemo-Mechanical Grinding (CMG) of Single Crystal Si Wafer

Chemo-mechanical grinding (CMG) process is a promising process for large-sized Si substrate fabrication at low cost. However, effect of additive in CMG wheel is not completely understood yet. In this paper, three different CMG wheels were developed, in which one excluded additive and the other two contained two kinds of additive i.e. silicon dioxide and sodium carbonate. Grinding experiments were conducted to explore the influence of exclusion of additive and inclusion of different kinds of additive on CMG performance. The grinding characteristics of the three wheels were also analyzed and discussed to reveal the roles of wheel compositions in CMG process. This work provides some fundamental insights for the selection of different types of additive for optimization of CMG wheel.

Effects of Sodium Carbonate and Ceria Concentration on Chemo-Mechanical Grinding of Single Crystal Si Wafer

Chemo-mechanical grinding (CMG) process is a promising process for large-sized Si substrate fabrication at low cost. An encountered issue in current CMG process of Silicon (Si) wafers is metallic contaminations on ground Si wafer surface, which is attributed to the existence of sodium carbonate in wheel compounds. In this paper, four different CMG wheels were developed and grinding experiments were conducted to study the effects of exclusion of sodium carbonate and concentration of ceria abrasive on grinding performance. The grinding characteristics of the four wheels were analysized and discussed to reveal the effects of different compositions.

Study on Improvement of Material Removal Rate in Chemo-Mechanical Grinding (CMG) of Si Wafer

Silicon wafer thinning process is meeting great challenges to fulfill requirements of ultra-thin IGBT for automotive applications. Chemo-mechanical grinding (CMG) process is potentially emerging stress relief thinning process which combines the advantages of fixed abrasive machining and chemical mechanical polishing (CMP). A major issue in CMG of Si wafers is the relatively low material removal rate (MRR). This paper studies the influence of the wheel specifications and grinding conditions on the MRR of CMG. Two sets of three-factor two-level full factorial designs of experiment (DOE)[1] are employed to reveal the main effects and interacted effects of CMG wheel specifications and grinding parameters on MRR. The optimal combination scenarios for improving MRR of CMG are analysized and obtained. By use of the optimal CMG wheel and grinding parameters, the MRR of more than 60nm/min is achieved.

Study on Subsurface Damage Generated in Ground Si Wafer

In this paper, we study and evaluate the subsurface damage of the ground wafers to understand the effect of residual stress on the wafer deflection. The experimental results show that two indexes of depth of the damaged layer and degree of the residual stress are directly associated with the warpage of wafer. The degree of the damage decreases with an increasing in grit size of diamond wheel. The theoretical analysis suggests that the minimally achievable thickness of wafer is proportional to the degree of the damage introduced by respective process.